The invention relates to a method and apparatus for producing a seam and is capable of stopping the formation of stitches precisely at a predetermined location.
The problem of producing a seam with an exact length always arises where two workpieces must be sewn to each other in a visible location and the course of the seam must not extend over the edge of the sewn-on workpiece; or in the production of a corner seam where the seam end must be equidistant from both workpiece edges.
In both cases, the formation of stitches must be ended at a location exactly definable in advance in order to produce a visually unobjectionable seam: in the first case, at the end of the sewn-on workpiece; in the second, at the point that is the same distance from both edges of the workpiece to be sewn.
Various devices intended to produce such seams are known.
German Unexamined Patent Application No. OS 30 18 797 discloses a sewing machine with a stitch-counter correction device which assures that the actual seam end deviates by a maximum of one-half stitch length from the desired seam end. Whether such deviation is in front of or beyond the desired end depends on what percentage of the stitch being formed is complete at the time when the approaching desired seam end is detected.
European Patent No. 0 068 625 discloses a stitch-length monitoring device for sewing machines. With this monitoring device it is possible to obtain a precision of one-fourth stitch in the desired seam length. Here too, the desired seam end is detected several stitches in advance, and a variable number of stitches to be made is calculated by a control means. The number of stitches still to be executed after detection can be increased by a half or a full stitch length, depending on the degree of completion of the stitch at the time of detection. The stated precision is achievable in conjunction with a reversal of the advance of the workpiece.
European Patent No. 0 102 524 discloses a drive and control device for sewing machines in which device the desired seam end is likewise detected several stitches in advance. A correction means varies the length of at least one of the remaining stitches as a function of the instantaneous position of the needle at the time of the detection of the end, so that the last point of insertion of the needle lies as close as possible to the desired end.
Here the degree of deviation of the actual end from the desired end depends on the number of remaining stitches and the possibility of varying their length. A correction of the stitch lengths is then practical only if the deviation from the desired end is greater than half of a correction stitch.
With none of these known devices is it possible to reliably position the final point of insertion exactly at the desired location.